In known methods and apparatus for mixing gaseous mixtures into the sea water of a protein skimmer there are problems due to the fact that the efficiency of flotation is highly dependent on the size of the gas bubbles used. Thus too small bubbles have an inadequate buoyancy and are entrained downwards by the water flow, whilst too large bubbles rise excessively rapidly and therefore either do not attach to the particles to be removed, or come loose from them again due to their excessively high rate of rise.
As the protein particles and the attached bubbles rise counter to the water flow direction to become removed from the circuit by the overflow of the contact tube, the protein skimmer operates most effectively when used in counter current manner, i.e., the water flows from top to bottom and the gas bubbles from bottom to top, i.e., oppositely to one another. The gaseous mixture required for skimming must be introduced as near to the bottom as possible over the contact tube outlet. In order to permit this, a vacuum or underpressure must be produced, which is higher than the hydrostatic pressure at the point at which suction of the gaseous mixture is to take place.
Hitherto, the air or air-gas mixture has been introduced into the contact tube of a protein skimmer by means of effusers made from lime or linden wood or other fine-pored materials through which compressed air is forced, or according to a second process by means of a centrifugal pump having an open, exclusively water-sucking impeller. In the second method, the gaseous mixture to be admixed with the water sucked by the centrifugal pump is sucked through the water flow from an annular duct, which is spaced from the impeller of the centrifugal pump and surrounds the impeller's periphery.
The first-mentioned process suffers from a poor reproducibility of the air or gas quantity, because in the case of the effusers the pores are either encrusted by salt or so widened by ozone that it is necessary to constantly check and readjust the air pressure and therefore air quantity and bubble size. Furthermore, for the purpose of the ozonization of the aquarium water, the air must be dried because otherwise condensation water is precipitated in the ozonizer and consequently the latter is no longer able to function.
In the case of the pump-operated protein skimmers, although air drying is made superfluous, the energy requirement is 550 to 1100 Watts and the amount of noise produced precludes operation in the home. The size, i.e., an external diameter of 0.5 to 1.0 m and a height of 2.53 to 3.4 m makes it possible to install such a protein skimmer in the home. The pump driving motors require a high power consumption. Due to the complicated operation and the resulting difficult and complicated manufacture, such a skimmer is also much to expensive for most aquarium operators.